This invention relates to an amorphous metal core transformer, and particularly relates to an amorphous metal core transformer capable of reducing core losses and watt losses.
An amorphous metal core transformer, which transforms A.C. power of a high voltage and a small amperage into that of a low voltage and a large amperage, or vise versa, using amorphous metal sheets as for a material of its magnetic core, is so popular nowadays. As for the magnetic core of the amorphous metal core transformer, a wound core or a laminated core is employed. The wound core is chiefly employed and it is formed by winding amorphous metal strips. For example, as disclosed in Japanese Patent Applications Nos. Hei 9-149331 (Japanese Patent Laid-open No. JP-A-10-340815) and JP-A-9-254494, an amorphous metal core transformer for three phase 1000 kVA use with five-legged core, employs wound cores and coils in a transformer casing. In actual designing of the transformer in these related arts, amorphous magnetic strips are wound to form a unit core of approximately 170 mm in width and approximately 16200 mm2 in cross-sectional area. Two unit cores are juxtaposed edgewise to compose a set of unit cores to increase (in this case, to double) the cross-sectional area. Four sets of unit cores are arranged side by side so as to compose a five-legged core. Three coils are combined with the five-legged core so as to compose the three phase transformer. The five-legged core has first leg, second leg, third leg, fourth leg and fifth leg arranged in this order. The coils consist of three coils, which are first coil, second coil and third coil and are inserted in the second leg, the third leg and the fourth leg respectively. Actual weight of the inner unit cores and outer unit cores are about 158 kg and about 142 kg respectively.
Coils in an amorphous transformer according to the related art, as shown in FIG. 4B, are composed of a primary coil 121 and a secondary coil 122 for three phases. The primary coil 121 uses a rectangular insulated copper wire measuring 3.5 mm×7.0 mm, having a conductor cross-sectional area of 24.5 mm2, which is wound 418 turns. The secondary coil 122 uses two parallel copper conductor strip having a conductor cross-sectional area of 603.5 mm2, which is wound 13 turns. The primary coil 121 is arranged outside the secondary coil 122 in the radial direction of the coil. In order to let out the heat generated inside the coils, duct space layers 24 are formed within the coils 2 for circulating insulation oil therein. In each of the duct space layers, a spacer members having a plurality of rod-shaped members 23 shown in FIG. 4C, is inserted so as to form a loop within the coil. Since the amorphous metal core transformer in the related art has large losses, a sufficient cooling capacity is required for the duct space layers 24. Accordingly, six duct space layers 24 are disposed both between the second leg and the third leg and between the third leg and the fourth leg. Since the duct layers 24 are formed in coaxial loops, both coil ends of the coil 2 is disposed facing the cores by narrow gaps, which impedes circulation of insulation oil.
In general, a transformer is designed in such a manner that the current density in the primary coil and that in the secondary coil are nearly equal as possible and, when different conductor materials are used for the two coils, the current densities calibrated by electrical resistances of the coils are also nearly equal. Further, as connection systems for three phase transformers, Y (star) connection and Δ (delta) connection are known. When the capacity of the transformer is small, Δ connection is disadvantageous because a greater number of turns are required than that required in Y connection. On the other hand, when the capacity of the transformer is in the medium range or above, Y connection is disadvantageous because a wider cross-sectional area of the conductor is required than that required in Δ connection. Therefore, in the small capacity range of 500 kVA or less, Y-Δ connection is used, and in the medium capacity of 750 kVA or more, Δ—Δ connection is mainly used. And in the latter, some transformers use Y-Δ connection. Where Y connection is used, it is possible to reduce the turns of the coil windings 1/√{square root over (3)} times to that in Δ connection. However, the amperage of the current flowing through the coil is the same value as that in Δ connection, which requires the same cross-sectional area of the coil conductor as that in Δ connection. On the other hand, though Δ connection requires the turns of the coil windings √{square root over (3)} times to that in Y connection, amperage of the current flowing through the coil is reduced to 1/√{square root over (3)} times to that in Y connection, which enables to reduce the cross-sectional area of the coil conductor.
An magnetic core-coil assembly, as shown in FIGS. 7 and 8 of the JP-A-10-340815, is composed of eight unit magnetic cores and three coils. The unit magnetic core has a joint portion in one of its yokes, and when this joint portion is opened, the core is formed into U-shape so as to be able to insert its legs into the coils. After insertion, the joint portion is closed and the magnetic core and the coil are assembled.
A transformer casing has a similar configuration to one shown in FIG. 3, which accommodates the magnetic core-coil assembly and insulating oil inside, and has external terminals, cooling fins outside. The external terminals are electrically connected to the coils through line wires. The cooling fins radiate the heat generated in the coils or magnetic cores and the heat transmitted to the insulating oil into the atmosphere to keep the temperature increase within an allowable range. The height of the cooling fins is designed to be approximately 100 to 200 mm. The total surface area of the cooling fins is supposed to be about 10 times as large as the surface area of the casing, and is designed to be approximately 50 m2.
In case of a conventional amorphous metal core transformer for three phase 1000 kVA use, total losses will amount to approximately 11730 W including core losses of approximately 330 W and watt losses of approximately 11400 W, which requires a large cooling area to keep the temperature increase within the allowable range. In addition, if loss reduction is attempted by reducing the watt losses so as to increase the conductor cross-sectional areas of the primary and secondary coils, it is necessary to use thicker, accordingly more rigid copper wires. This makes the winding work more difficult due to rigidity of the wires, and in addition, connection between the secondary coil and the line wire becomes more difficult, which deteriorates productivity requiring more man-hours.